refactor: remove blocking RPC call from hot path

CRITICAL FIX: Eliminated blocking RPC call in reader.go that was fetching
transaction data we already had from the sequencer feed.

Changes for consistency and reusability:
1. Added RawBytes field to DecodedTransaction to store RLP-encoded transaction
2. Created reusable ToEthereumTransaction() method for type conversion
3. Changed channel from 'chan string' (txHashes) to 'chan *SwapEvent' (swapEvents)
4. Updated processSwapEvent to use transaction from swap event instead of RPC

Impact:
- REMOVES blocking RPC call from hot path (pkg/sequencer/reader.go:357)
- Eliminates network latency from transaction processing pipeline
- Uses data already available from Arbitrum sequencer feed
- Improves throughput and reduces RPC dependency

This fixes the #1 CRITICAL blocker for production deployment identified in
PRODUCTION_READINESS.md.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

pkg/sequencer/decoder.go

@@ -43,6 +43,7 @@ type DecodedTransaction struct {
 	Nonce    uint64
 	GasPrice *big.Int
 	GasLimit uint64
+	RawBytes []byte // RLP-encoded transaction bytes for reconstruction
 }
 
 // DecodeArbitrumMessage decodes an Arbitrum sequencer feed message
@@ -145,11 +146,27 @@ func DecodeL2Transaction(l2MsgBase64 string) (*DecodedTransaction, error) {
 		Nonce:    tx.Nonce(),
 		GasPrice: tx.GasPrice(),
 		GasLimit: tx.Gas(),
+		RawBytes: txBytes, // Store for later reconstruction
 	}
 
 	return result, nil
 }
 
+// ToEthereumTransaction converts a DecodedTransaction back to *types.Transaction
+// This is a reusable utility for converting our decoded format to go-ethereum format
+func (dt *DecodedTransaction) ToEthereumTransaction() (*types.Transaction, error) {
+	if len(dt.RawBytes) == 0 {
+		return nil, fmt.Errorf("no raw transaction bytes available")
+	}
+
+	tx := new(types.Transaction)
+	if err := rlp.DecodeBytes(dt.RawBytes, tx); err != nil {
+		return nil, fmt.Errorf("failed to decode transaction: %w", err)
+	}
+
+	return tx, nil
+}
+
 // IsSwapTransaction checks if the transaction data is a DEX swap
 func IsSwapTransaction(data []byte) bool {
 	if len(data) < 4 {